A bipolar junction transistor (BJT) is a three-terminal electronic device constructed of doped semiconductor materials, which may be used in amplifying or switching applications. The operation of bipolar junction transistors includes both electrons and holes. Charge flow in a BJT is due to bidirectional diffusion of charge carriers across a junction between two regions of different charge concentrations. The mode of operation of a BJT is contrasted with unipolar transistors, such as field effect transistors, in which only one carrier type is involved in charge flow due to drift. By design, most of the BJT collector current is due to the flow of charges injected from a high-concentration emitter into the base where there are minority carriers that diffuse toward the collector.
Conventional epitaxial semiconductor growth of the doped regions of a BJT typically requires high temperatures (generally much greater than 600° C.). Depending on the application, the high epitaxial growth temperature may have any or all of the following drawbacks: degradation of minority carrier lifetime, creation of structural defects, undesired impurity diffusion resulting in junction widening, relaxation of strain, or generation of undesirable strain resulting in buckling or delamination.